1. Field of the Application
The present invention relates to metallurgy, and more particularly to a method of making billets for subsequent rolling at bar-, and tube-rolling mills.
The invention is readily adapted for use where billets are produced by continuous casting of metal, which is either followed by, or combined with, the rolling of said billets from high-alloy steels and alloys.
2. Description of the Prior Art
It is modern practice to produce billets by the method of continuous casting of semi-finished products. It should be observed that along with structural stability and chemical homogeneity of the continuously-cast billet over its length, it also has a developed area of axial porosity and segregation of chemical constituents in the transverse section thereof. At present, there are no methods known to have sufficient effect on the metal in the process of continuous casting to prevent axial segregation. The central portion of the continuously-cast billet is of special importance for the production of individual billets separated therefrom, the performance of which depends on the extent of axial segregation developing, for example, in the process of making ball-bearing-, tool-, and corrosion-resistant steels. The preservation of axial segregation in the rolled sections and bars made of high-alloy tool steels renders them unsuitable for the application where macro, and microstructure is under stringent control, the segregation may be caused, for example, by non-uniformity of carbide. Non-uniform distribution of carbon and alloying components in the finished product will result in the non-uniform distribution of carbide phase. Local concentration of carbides and the carbide banded structure greatly impair the operational properties of the finished product. If carbide has banded structure, the cutting edge of a cutting tool tends to become friable, which adversely affects its durability.
It should be noted, however, that the method of continous casting is far more effective than other conventional methods of making billets. Thus, it is essential to develop a method for the production of billets free from macrodefects due to shrinkage, which will make it possible to prolong durability and improve reliability of the finished product at reduced production cost, the features long anticipated by both producers and consumers of high-quality steels.
There is known a method of making billets for special-purpose articles which demand perfectly homogeneous and dense metal. According to this method, a casting is produced in a mould and is then subjected to rolling on a roughing mill, whereafter the riser portion is cut off.
The method referred to above is disadvantageous in that it entails an appreciable waste of metal, above 20 percent, plus additional expences required for making the casting into a billet. Therefore, the production cost of the finished product is rather high.
For the most demanding applications electroslag melting methods are resorted to, though in this case the production cost of metal raises 2 to 2.5 times as compared to the method described above.
Another widely practiced method of making special-purpose billets consists in that metal-cutting lathes are used to remove a developed area of axial porosity and segregation. The billet is separated in the direction of its length into two or three sections, with the area of axial segregation being then removed therefrom by planing. This method is alternatively used for the production of hollow articles, for example, such as high-pressure boilers.
This being the case, the axial portion of the billet is drilled out and the billet is then drawn over mandrels. Needless to say that the mechanical operations required to remove axial porosity and the ensuing segregation substantially add up to the production cost of the finished product.
There is known a more advanced method of continuous casting, wherein the continuous casting is subjected to deformation while having its core still unsolidified.
The disadvantage of the above-mentioned method lies in the impairment of the quality of the cast product due to an increasing concentration of segregates as well as in the occurrence of defects resulting from the disruption of the continuity during reduction of the casting in a two-phase state.
There is known a method of improving the quality of the finished product, according to which a slab is rolled in multi-grooved roll passes, whereby a segregation zone is localized in the middle billet. The extreme billets are free from segregation.
However, the production of good quality billets according to the method described above is accompanied by substantial waste of metal approximating 33 percent, since the middle billet is only suitable for remelting.
The prior-art method of making billets for a cutting tool has found but a limited application. This method consists in producing a casting of rectangular cross section with the ratio of its sides being 1:2, said casting being thereafter separated in the direction of its length and divided into two equal sections. Each of the two sections of the casting is then subjected to working.
The above-mentioned method suffers from a disadvantageously high operating cost due to low production efficiency thereof caused by the cutting operation, as well as considerable waste of metal.
Widely employed in continuous casting machines are multistrand and multipath moulds. By casting metal in the moulds of such construction, billets of planar structure are formed therein which are separated from one another and connected to the central portion of the casting. Resulting therefrom is a star-shaped composite billet.
The amount of metal waste (the central portion) is 15 percent of the overall weight of the casting; in addition, there is no guarantee for the flawlessness of the individual billets severed from the central portion of the casting.
There is known a continuous casting mould which comprises cooled walls defining the mould cavity star-shaped in profile. The mould of this type is used for producing a composite billet star-shaped in cross section and having ray members spaced symmetrically in relation to the central axis.
The disadvantage of the known mould construction lies above all in the formation of the continuous casting, which, when severed into individual billets, results in the lower production output of the finished product free from defects in the axial area thereof. In view of the fact that the width of the base of each section of the mould cavity is larger at the place of juncture with other sections than the oppositely spaced base, the longitudinal separation of the individual billets from the casting is difficult to carry out and is accompanied by substantial waste of metal. With this method it becomes possible to bring the yield of good billets free from macrodefects due to shrinkage only up to 70 percent.